Synthesis gas (“syngas”) is a gas mixture comprising primarily carbon monoxide hydrogen of varying amounts and with some carbon dioxide. Syngas is widely used as a reactant for industrial chemicals. There are a variety of methods to purify syngas to produce different product purities of carbon monoxide and hydrogen. Syngas separation is energy intensive and one of the most expensive process steps to obtain pure hydrogen and carbon monoxide for industrial use. Cryogenic purification is widely used to separate syngas as described in U.S. Pat. Nos. 5,511,382, 4,756,730, and 4,242,875. One problem in producing pure hydrogen and carbon monoxide streams is that the separation is energy intensive. The presence of additional gases, such as nitrogen and methane, may further increase the energy requirements for separation. Also, the complexity of the purification process is dependent on the desired purity of the carbon monoxide and hydrogen.
Some conventional processes have used syngas directly in producing industrial chemicals, as described in U.S. Pat. No. 6,596,781, which shows using syngas for a Fischer-Tropsch process to produce diesel, and syngas for producing methanol using a copper and/or zinc catalyst. Methanol synthesis may be highly selective, but the process consumes both hydrogen and carbon dioxide from the syngas to produce methanol. Although there may be effluent gas from the methanol synthesis process, the composition may vary depending on the hydrogen to carbon monoxide ratio of the syngas and selectivity to methanol. Thus, these processes cannot separate syngas.
Various processes have been described for producing acetic acid from syngas. For example, U.S. Pat. No. 8,088,832 describes a method for synthesizing ethanol using synthetic routes via syngas. A method and apparatus for gasifying biomass in a steam gasifier that employs a fluidized bed and heating using hot flue gases from the combustion of syngas is described. Methods and apparatus for converting syngas into ethanol are also disclosed, using stepwise catalytic reactions to convert the carbon monoxide and hydrogen into ethanol using catalysts including iridium acetate. The stepwise catalyst reaction converts methanol, carbon monoxide, and hydrogen into a mixture comprising methyl acetate, hydrogen, methanol, acetic acid, and water. Hydrogen and methyl acetate are separated from the mixture and used in the ethanol production.
U.S. Pat. No. 8,080,693 describes processes for converting methanol to ethanol by reacting methanol and carbon monoxide in the presence of a catalyst to produce a product having at least 25 mole % methyl acetate and, in some instances, acetic acid. The acetic acid is then reacted with at least one alcohol to produce at least one acetate selected from methyl acetate, ethyl acetate, and butyl acetate. The at least one acetate and the methyl acetate produced as a result of reacting methanol and carbon monoxide then are hydrogenated to produce ethanol. Syngas may be produced from biomass to produce all or a portion of the methanol, hydrogen, and carbon monoxide requirements for the process. Hydrogen is separated from the syngas prior to the carbonylation reactor. A hydrogen permeable membrane is described for producing a carbon monoxide stream having less than 5 mol. % hydrogen.
U.S. Pat. No. 7,498,016 describes a method for the production of syngas from methanol feedstock. The methanol feed is supplied to a partial oxidation reactor with oxygen and optionally steam to yield a mixed stream of hydrogen, carbon monoxide, and carbon dioxide. The carbon dioxide is separated out and the hydrogen and carbon monoxide mixture is fed to a cold box where it is separated into hydrogen-rich and carbon monoxide-rich streams. The separated carbon dioxide can be recycled back to the partial oxidation reactor as a temperature moderator if desired. The carbon monoxide-rich stream can be reacted with methanol in an acetic acid synthesis unit by a conventional process to produce acetic acid or an acetic acid precursor. Optionally, an ammonia synthesis unit and/or vinyl acetate monomer synthesis unit can be integrated into the plant.
U.S. Pat. No. 6,596,781 describes an integrated process for carrying out the production of Fischer-Tropsch products and acetic acid made using the methanol and carbonylation route which uses the hydrogen recovered from the methanol production to upgrade the Fischer-Tropsch products.
U.S. Pat. No. 5,659,077 describes an integrated process for production of acetic acid that involves subjecting a feed mixture consisting of (a) methane gas and (b) gaseous oxygen, air, or a mixture thereof, to partial oxidation without production of syngas in a reaction zone at elevated temperature and pressure to form a reaction mixture containing methanol, carbon monoxide, carbon dioxide, methane and water vapor. At least a portion of the water vapor is removed from the reaction mixture, and the remaining partial oxidation reaction mixture is fed, together with additional methanol from an external source, through a carbonylation reaction zone at elevated temperature and pressure to form a reaction product containing acetic acid and/or methyl acetate and methanol. The additional methanol is added in an amount such that the additional methanol together with the methanol produced by partial oxidation is sufficient to convert substantially all of the carbon monoxide produced by partial oxidation. Excess methane and carbon dioxide are recycled from the carbonylation reaction zone back to the partial oxidation reaction zone, and methanol in the carbonylation reaction product is recycled back to the carbonylation reaction zone and acetic acid and/or methyl acetate is recovered as product.
WO2003097523 describes a process that produces both methanol and acetic acid under substantially stoichiometric conditions, wherein an unadjusted syngas having an R ratio less than 2.0 is provided. All or part of the unadjusted syngas is supplied to a separator unit to recover CO2, CO and hydrogen. At least a portion of any one or combination of the recovered CO2, CO and hydrogen is added to any remaining syngas not so treated or alternatively combines in the absence of any remaining unadjusted syngas to yield an adjusted syngas with an R ratio of 2.0 to 2.9 which is used to produce methanol. Any recovered CO2 not used to adjust the R ratio of the unadjusted syngas can be supplied to the reformer to enhance CO production. At least a portion of the recovered CO is reacted in the acetic acid reactor with at least a portion of the produced methanol to produce acetic acid or an acetic acid precursor by a conventional process.
Other processes describe converting syngas to ethanol. U.S. Pat. No. 7,718,832 describes a catalytic process that selectively produces ethanol by contacting syngas, composed primarily of hydrogen and carbon monoxide, with three catalysts within a reactor. The first catalyst is a hydrogenation promoter comprising Cu—Zn, Mo or Fe with an optional alkali metal additive and an optional support of aluminum oxide, silica, zeolite or clay. The second catalyst is a homologation promoter comprising one or more of the Group VIII metals in free or combined form with a co-catalyst metals consisting of Y or lanthanide or actinide series metals with optional additives and support. The third catalyst is a hydrogenation promoter.
U.S. Pat. No. 7,842,844 describes a process for the conversion of hydrocarbons to C2 oxygenates, and uses a conventional catalyst to hydrogenate the C2 oxygenate feed. Hydrocarbons are converted to ethanol and optionally acetic acid by converting hydrocarbon in a syngas reactor into a stream A comprising a mixture of carbon oxide(s) and hydrogen preferably having a H2/CO molar ratio between 1.5 and 2.5, converting at least part of stream A in the presence of a particulate catalyst in a reactor under a temperature between 150 and 400° C. and a pressure of 5 to 200 bar, into a C2-oxygenates stream B, where stream B includes water, alkanes, ethanol, acetaldehyde, ethyl acetate and acetic acid, which together represent at least 80% by weight of the products obtained from the C2-oxygenates conversion reactor. The C2-oxygenates stream B is separated into a stream C comprising H2, CO, CO2 and alkanes, and a stream D including 15 to 40 wt. % acetic acid, 10 to 40 wt. % acetaldehyde and 15 to 40 wt. % ethanol. At least part of stream D is hydrogenated in a hydrogenation reactor into an ethanol stream E, and stream E is subjected to a separating step, followed by recovery of ethanol.
U.S. Pat. No. 8,502,001 describes a process for the production of ethanol from a carbonaceous feedstock, wherein the carbonaceous feedstock is first converted to syngas which is then converted to ethanoic acid, which is then subject to a two stage hydrogenation process by which at least a part of the ethanoic acid is converted by a primary hydrogenation process to ethyl ethanoate, which ethyl ethanoate is converted by a secondary hydrogenation process to produce ethanol.
EP02060553 describes a process for converting hydrocarbons to ethanol involving converting the hydrocarbons to ethanoic acid and hydrogenating the ethanoic acid to ethanol. The stream from the hydrogenation reactor is separated to obtain an ethanol stream and a stream of acetic acid and ethyl acetate, which is recycled to the hydrogenation reactor.
Thus what is needed is a process for separating syngas that significantly reduces capital costs and is energy efficient.